1. Field of the Invention
This invention relates to semiconductor fabrication processes, and more particularly, to a method for fabricating a semiconductor read-only memory (ROM) device which is used for permanent storage of multi-level coded data therein.
2. Description of Related Art
Read-only memory (ROM) is a nonvolatile semiconductor memory widely used in computer and microprocessor systems for permanently storing information including programs and data that will be repeatedly used. The manufacture of ROMs involves very complicated and time-consuming processes and requires costly equipment and material to achieve. Therefore, the data that are to be permanently stored in ROMs are first defined by the customer and then furnished to the factory to be programmed into the ROMs.
Most ROMs are identical in semiconductor structure except for the different values of data stored therein. Therefore, the ROM devices can be fabricated up to the stage ready for data programming and then the semi-finished products are stocked in inventories waiting for customer orders. The customer then furnishes the data to the factory where the data are stored into the semi-finished ROMs by using, the so-called mask programming process. This procedure is presently a standard method in the semiconductor industry for fabricating the ROMs.
Conventional ROMs are usually based on metal-oxide semiconductor field-effect transistor (MOSFET) memory cells, each memory cell being used for the storage of one value of the binary-coded data. In the mask programming process, these MOSFET-based memory cells are selectively doped with impurities into the associated channel regions so as to vary the threshold voltage thereof. This can set a first selected group of the memory cells to a permanently-ON state representing the permanent storage of a first value of the binary-coded data, for example 0, and a second selected group of the memory cells to a permanently-OFF state representing the permanent storage of a second value of the binary-coded data, for example 1.
FIG. 1 shows a schematic top view of a conventional MOSFET-based ROM device which is specifically designed for permanent storage of binary-coded data therein. This ROM device includes a plurality of memory cells as exemplarily indicated by the dashed boxes labeled with the reference numerals 100, 101, and 102, which can be accessed via a plurality of intercrossed word lines WL and bit lines BL. Each of the memory cells is associated with one segment of the word line between each neighboring pair of the bit lines under which one channel region, as exemplarily indicated by the reference numeral 10, is formed. Whether the memory cell 100 stores the binary data 0 or 1 is dependent on whether its associated channel region 10 is doped with impurities or not.
The foregoing type of ROM device is specifically devised for permanent storage of binary-coded data which have two different values only. To allow for a larger capacity of data storage, the data can be represented by more than two values. For example, a ternary system handles the data by representing them in three different values, and a quaternary system handles the data by representing them in four different values. In general, such a system is referred to as an M-ary system for handling multi-level coded data represented by M different values (M&gt;2). For permanent storage of the multi-level coded data, the memory cells of ROM device should be selectively set to M different threshold voltage levels which represent the M different values of the multi-level coded data. In fabricating such a ROM device, the so-called multiple code-implantation process is used to form the memory cells with various threshold voltage levels representing the permanent storage of different values of the multi-level coded data.
For instance, as shown in FIG. 2, in a ROM device for permanent storage of multi-level coded data, the channel regions of the MOSFET-based memory cells are doped in a different manner so as to provide a first threshold voltage V.sub.T representing the permanent storage of a first value of the multi-level coded data, a second threshold voltage V.sub.T1 representing the permanent storage of a second value of the multi-level coded data, a third threshold voltage V.sub.T2 representing the permanent storage of a third value of the multi-level coded data.
One major drawback to the foregoing ROM device, however, is that the multiple code-implantation process requires the ion-implantation process to be performed for several times in different conditions. The mask programming process is thus very time-consuming to carry out. The delivery period of the product of the ROM device is thus long, which makes the ROM device less competitive in the market.